Climate change has become a growing concern over the past years and much research has been directed to the provision of new energy sources and efficient means for storing and converting energy. Among the energy storage and conversion devices investigated, lithium oxygen batteries, also known as lithium air batteries, have been found to present many advantages over conventional electrochemical cells and lithium batteries. Lithium oxygen (Li/O2) batteries can use oxygen from the air instead of storing an internal oxidant. Due to the very high specific capacity of the oxygen cathode and lithium anode, lithium oxygen batteries have attracted much interest from scientists both in industry and in academia. For example, the practical specific energy of lithium oxygen batteries is 500 W h kg−1, which is 2 to 3 times higher than that of the best lithium-ion batteries available today on the market.
Li/O2 batteries are comprised of a lithium anode, a cathode and an electrolyte. The oxygen redox reaction (ORR) takes place at the solid cathode/electrolyte interface. The solid cathode is a carbon/catalyst layer which is fed by oxygen; the anode is typically a Li metal foil. In non-aqueous, aprotic Li/O2 batteries, the cathode reaction involves the formation of superoxide, peroxide and oxide species of lithium on the cathode surface. During recharge, such species are expected to be reoxidized to give oxygen gas. The low conductivity of solid lithium peroxide and solid lithium oxide causes electrode insulation, affects cathode discharge capacity and determines high cathode recharge overpotentials. While cathode discharge potentials are approximately 2.5-2.7 V vs. Li+/Li, recharge potentials are typically higher than 3.5 V vs. Li+/Li. This makes the choice of electrolytes crucial.
While carbonate and ether based electrolytes are not stable towards oxygen radical species or to the high potentials required for recharge, dimethyl sulfoxide, ionic liquids and tetraethylene glycol dimethyl ether (TEGDME) based electrolytes are considered to be good alternatives. To date, two types of non-aqueous Li/O2 battery configurations have been reported: air breathing cells, where the cathode is in direct contact with gaseous oxygen (or air) and flow cells, where the electrolyte is fed with oxygen.
Redox flow batteries are another type of battery and have a two electrolyte system in which the two-electrolytes, acting as liquid energy carriers, are pumped simultaneously through the two half-cells of the reaction cell separated by a membrane. The membrane allows the passage of ions and thus facilitates ion exchange between the two electrolytes. Electrons which are released in this process are able to move around the external circuit and do work. Electrolyte in flow batteries is stored externally to the cell and is pumped through the cell. It can be replaced and thus the battery can be “recharged” and any spent material reenergised.
Mihai Duduta et al. (Adv. Energy Mater., 2011, 1, 511-516) report a semi-solid lithium-ion rechargeable flow battery. This flow battery consists of positive and negative electrodes composed of particles suspended in a carrier liquid. The particles flow in the cathodic or anodic compartments which are separated by ion-conducting membranes. The flowable cathode may comprise Li compounds such as: LiCoO2, LiFePO4, and LiNi0.5Mn1.5O4, and the flowable anode may comprise compounds such as Li4Ti5O12, graphite, and Si. The electrolyte may also comprise Ketjen black as a dispersed conductive phase. Energy is stored in the suspensions of the flowable cathode and anode and the transfer of charge to current collectors takes place via percolating networks of nanoscale conductors (e.g. Ketjen black).
Simone Monaco et al. (J. Phys. Chem. Lett., 2013, 4, 1379-1382) found that oxygen electrode response in ionic liquids at high discharge currents is dominated by oxygen mass transport in ionic liquids. To solve this problem Simone Monaco et al. proposed to bubble oxygen into the electrolyte (N-butyl-N-methyl pyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR14TFSI):LiTFSI 9:1) which continuously circulated through the cathode compartment. A flow-cell fed with oxygen saturated ionic liquid electrolyte had a discharge capacity of up to 600 mAh g−1 under discharge currents of 0.2 mA cm−2, with a recharge efficiency of 92%.
Yun Guang Zhu et al. (Chem. Commun., 2015, 51, 9451-9454) reported a flow lithium oxygen battery with soluble redox catalysts that comprises a lithium metal anode separated from a carbon felt cathode by a membrane; and a gas diffusion tank connected to the cathodic compartment by a pump. Electrolyte (such as lithium bis(trifluoromethane)sulfonimide in tetraethylene glycol dimethylether) with soluble redox catalysts is circulated between the gas diffusion tank and the cell. During the discharging process, oxygen flows into the gas diffusion tank, and is reduced to form Li2O2. This is then deposited on the porous matrix of the tank. The gas diffusion tank thus assists in preventing passivation and pore clogging of the cathode. The authors of the paper postulate that the capacity of the cell would be limited by the size of the gas diffusion tank.
Volker Presser et al. (Adv. Energy Mater., 2012, 2, 895-902) report an electrochemical flow capacitor which functions by storing energy in an electric double layer of charged carbon particles. A flowable, semi-solid carbon-electrolyte mixture is employed as the active material for capacitance energy storage. When this electrolyte mixture is pumped between two polarised current collectors, an electric double layer forms at the surface of the carbon particles. The positively charged solid particles in the slurry attract negatively charged ions for charge balancing, and the negatively charged solid particles attract positively charged ions. Ion diffusion then occurs from one slurry electrode to the other through a membrane acting as an electrical insulator. The charged slurry can then be stored in reservoirs until the stored energy is needed.
Despite the advances in lithium batteries in recent years, there still exists a need for the production of lithium batteries with high discharge capacities, which are space efficient and inexpensive. Such batteries could be used in electric vehicles and in renewable energy plants.